1. Field of the Invention
The present invention relates to an image processing apparatus, a method for controlling the same and a storage medium.
2. Description of the Related Art
In recent years, advances have been made in the miniaturization of pixels of image sensors. The miniaturization of pixels in some cases causes noise to increase. Various methods to suppress the noise through signal processing are known, but it is known that suppressing noise causes a blurring effect on images.
As a method to avoid this, a method is known (Japanese Patent Laid-Open No. 2008-015741) that suppresses noise by dividing an image signal into a plurality of frequency band components (color components).
Further, a method is known (Japanese Patent Laid-Open No. 2009-199104) that suppresses noise by generating a reduced image signal obtained by reducing an image signal, and mixing the reduced image signal with the original image signal. According to this method, first by performing reduction processing on an inputted image signal a reduced image is generated, which contains low-frequency components to a greater extent than the inputted image. Next, an edge strength is detected from the low-frequency component reduced image signal, in order to determine, based on the edge strength, regions in which an edge component should be preserved. Then, a new image signal is generated by mixing the original image signal with the low-frequency component reduced image signal, while changing the weighting from region to region in such a way that images in the regions in which the edge component should be preserved do not blur.
However, when in the noise suppression processing described above the measures regarding image blurring are insufficient, increasing the amount of noise suppression leads to problems such as artifacts occurring, blurred noise patterns giving a visually uncomfortable feeling, or object microstructures that must not be removed being removed together with the noise component, which causes the image to become unnatural.
As a method to avoid this, a method is known (Japanese Patent Laid-Open No. 2001-008038) that, by capturing with a multi-band camera, uses the fact that there is a correlation between the images of different colors to adaptively add an edge component and a noise suppression component to a smoothed image. Moreover, a method is also known (Japanese Patent Laid-Open No. 2007-028348) that, after having frequency-separated an image, eliminates noise from the high frequency component, to later add a predefined noise before mixing it with the low-frequency component. To some extent, these methods avoid that visually unnatural images result from noise components being residual in images that have undergone smoothing.